Electrical prospecting



March 20, 1934. T. ZUSCHLAG I 1,951,386

ELECTRICAL PROSPECTING Filed Oct. 1, 1930 2Sheets-Sheet 1 ATTORN EY5 March 20, 1934. "T. ZUSCHLAG 1,951,386

ELECTRICAL PROSPECTING I Filed Oct. 1, 1930 2 Sheets-Sheet 2 ATTORNEYS Patented Mar. 20, 1934 cameo srarss PATENT ,QFFICE 1,951,386 ELECTRICAL PRO SPECTING Application October 1, 1930, Serial No. 485,638 16 Claims. (Cl. 175-182) This invention relates'to electrical prospecting and has for its object certain improvements in the method of electrical prospecting. The invention relates more particularly to an improved method that may be employed for determining and comparing electric potentials or electro magnetic field strengths, such as are encountered when investigating artificial electric ground fields set up for the purpose of making geo-physical explorations on the earth's surface and the like.

Artificial ground fields may be produced in areas or tracts to be investigated by direct cu"- rent or alternating current excitation. When direct current is used, the investigation of the ground field need be made only in respect to its potential or field strength. When alternating current is used, on the other hand, an investigation to be complete must in addition include also a determination of the time or phase relation between the exciter field, set up to induce the ground field, and the ground field itself. This is the reason why methods and apparata heretofore designed for direct current exploration cannot generally be used for alternating current exploration, especially if a neutralizing or compensating principle is involved; which appears highly necessary in order to obtain optimum results. Even with respect to methods and apparata heretoiore proposed for direct or alternating current exploration, serious difiiculties exist that complicate the analysis of the resultant readings. These difficulties concern the elimination of -resistances that are not necessary for the final analysis, as for instance ground contact resistances, or internal instrument resistances, the values of which vary for different settings of the apparatus employed.

As a result of my investigations I have determined that these difiiculties may be substantially overcome by the use of a novel method of conducting the geophysical. exploration. The method permits the determination of potential drop, or electro-magnetic field strength, ratios, and eliminates by the same operation substantially all unnecessary or interfering ground contact, or internal instrument, resistances.

In general, the method of the present invention is carried out by creating an artificial electric or electro-magnetic ground field in a tract of land and by determining the ratio between the electric potentials or the electro-magnetic field strengths created by such ground field at spaced points in the tract. In accordance'with the invention, this determination of the ratio between the potentials or electro-magnetic field strengths is carried out in such a manner as to eliminate the resistance of ground electrodes, or other pick-up means, changes in the strength of the ground field or variations in the resistance of the instruments employed, etc.

The potential or held strength ratios are con veniently determined from the flow of current produced in a conductive net-work including suitable variable resistances and inductances and current responsive means, the potentials or field strengths at the points under investigation being impressed on the net-work through suitable pickup means, such as electrodes conductively grounded in the tract, or receiving coils inductively associated with the tract.

The above mentioned resistances are eliminated by making two separate determinations of the potential or field strength ratios for each setting of the pick-up means, and by altering known variable resistances in the net-work to give a difierent value of current therethrough for each of these determinations. When the potential or field strength ratios are calculated from the data-obtained by these two determinations, factors such as ground electrode resistance, receiving coil impedance, instrument resistance, and ground field variation maybe eliminated and an accurate derivation of the desired ratios obtained. Since the ground resistances, receiving coil impedance instrument resistance and ground field strength do not always remain constant at different locations of the pick-up means, the elimination of these factors from the ratio determinations is essential if the results obtained at difierent settings are to be properly co-relatedand relatively analyzed.

In the practice of the present invention, spaced exciter or power electrodes connected to a source or" current supply are suitably grounded in a tract to be investigated. The power electrodes may be spaced several hundred or thousand feet apart. The source of current supply may be either an alternating or a direct current generator. -This field excitation arrangement is adapted to set up a ground field in the tract.

A conductive net-work is used in conjunction with the excitation arrangement, which is provided with suitable pick-up means to be placed and spaced within a ground field caused by the field excitation. The pick-up means may consist of potential electrodes, or receiving coils. These pick-up means are'suitably spaced from one another and from one of the power electrodes, so that a current derived. from the ground field is made to flow through the conductive network.

According to the present invention, in the case of the use of potential electrodes, three potential electrodes, or their equivalents, are grounded, preferably in a straight line, within the infiuence of one of the power electrodes. The center potential electrode is so placed that it will have a potential drop higher or lower than the two end potential electrodes. In order to provide a higher potential chapter the center electrode,

6 investigatedmay be such that the center potenthem must be placed in opposition to the other so that current induced in the coils from the following description, in which;

tial electrode may have a higher potential drop than either of the two end electrodes, although one of the end electrodes is placed nearer to the power electrode.

As will be more fully shown below, the object of providing the center potential electrode with a potential drop higher than that of the two end.

electrodes, is to make certain that current flows through the center electrodev to the end electrodes; or, in case the center potential electrode has a potential drop lower than that of the two end potential electrodes, that current will flow from the end electrodes to the center electrode. In each case current flows through the net-work in opposite directions;

It will thus be seen that the center potential electrode is merely one of reference, the aim being to obtain potential drop variations at the end electrodes. Such potential drop variations may be calculated from the data obtainable in the practice of the invention.

An artificial potential drop may be substituted for the center electrode, or for one of the end electrodes; by the use of special means to be discussed below. These artificial potential drops may be obtained. by various combinations, and. are highly useful in making certain investigations. I

In case induction coils are employed as pickup means in the conductive net-work, one of ground field may flow in opposite directions. To that extent the use of receiving coils is substantially similar to that of potential electrodes.

The present invention will undoubtedly be better understood if reference is made to the accompanying drawings, taken in conjunction with,the

Fig. 1 is a diagrammatic representation of apparatus adapted for the practice of the invention;

Fig. 2 is a diagrammatic representation of a modified form ,of apparatus adapted for the practice of the invention;

Fig. 3 is a diagrammatic curve indicating results obtainable when investigating an area containing a conductive ore body or the like with apparatus shown in Figs. 1 and 2.

Referring to Fig. 1, an alternating current source 1 is connected to grounded power or exciter electrodes 2 and 3, which electrodes may be separated several hundred or thousand feet. This arrangement is adapted to set up the necessary'ground field in a tract to be investigated.

The field excitation arrangement is used in conjunction with a compensating or neutralizing network consisting of non-inductive variable resistances 4, 5 and 6 connected in series with each other and with stators 7 and 8 of a variometer 9. The stators are in reverse position to one another. The free ends of the stators are connectd by means of a triple-pole double-throw switch 10 either to spaced and grounded potential electrodes 11 and 12, or to the free ends of pickup coils 13 and 14,-consisting of the same number of turns of insulated copper wire. One of the coils must be oppositely arranged to the other coil in order that currents induced in the coils may be made to flow in opposite directions.

The triple-pole double-throw switch further connects with a center potential electrode 15, or

the common ends of the pick-up coils, to the rotor 16 of the variometer. The rotor is connected in series to ,a common point of two oppositely exance 5 and the reversed stator 8 to the end po-.

tential electrode 12.

It will be noted that the net-work employed comprises two parallel circuits at and b; the circuit 0. including the variable resistance 4; and the circuit b including the variable resistance 5. The current responsivedevice 19 is included in the branch common to the two circuits, and the resistance 6 may be included in either of the circuits, depending upon the position of the switch 20. The resistance 6 is essentially an auxiliary resistance to increase resistance 4 or 5.

Referring to the modified form of apparatus shown in Fig. 2, it will be noted that the current source 1 is shunted by a potentiometer 21; a sliding contact arm 22 of which is connected to a pole of a single-pole double-throw switch 23. The otherpole of the switch is connected to the end potential electrode 11.

The sliding contact arm 22 also connects with a pole of a single-pole double-throw switch 24, the other pole of which is connected with the end potential electrode 12.

In operating the apparatus, the single-pole double -throw switches 23and 24 are so thrown that the sliding contact arm 22 connects either withend potential electrode 11 or end potential electrode 12. The two potential electrodes 11 and 12 should not simultaneously be connected with the potentiometer 21 when making observations on a tract under' investigation.

A further sliding contact arm 25 on the potentiometer connects with a pole of a single-pole double-throw switch 26, the other pole of which is connected to the center or auxiliary potential electrode 15. This arrangement may be used alone, or in combination with either electrode 11 or 12 in the apparatus just described above.

In the arrangement shown in Fig. 2, it will be noted that sliding contact arm 22, or the end potential electrode 11, may be connected in series with stator l of the variometer 9. The sliding contact arm 22, or the end potential electrode 12, maybe connected in series to the reversed stator 8 of the variometer. Furthermore, sliding contact arm 25, or the center potential electrode 15, may be connected in series with rotor 16 of the variometer.

In the apparatus shown in both Figs. 1 and 2,

it will be noted that the center potential electrode the direct influence of the other power electrode.

The end potential eletcrodes 11 and 12 are suitably and symmetrically spaced from both the center potential electrode 15 and the power electrode 2, so that the potential at the center electrode may be higher than that at the end electrodes. When the electrodes are placed in such relative positions, current will flow from the center electrode 15 to the end electrodes 11 and 12.

As indicated above, the three potential electrodes may be so placed relative to one of the exciter or power electrodes that the center or reference potential electrode will have a potential drop lower than that of the-two end potential electrodes. In such a case, current will flow from the end potential electrode to the center potential electrode.

Under the operating conditions just outlined, it will be apparent that whether the center potential electrode has the highest or the lowest potential drop, the current flowing through the circuits a and b will always be in opposite directions. In the present preferred practice of the invention, it is aimed to place the center potential electrode in such relation with respect to the power electrode 2 and the end potential electrodes 11 and 12, that it will have the higher potential drop. Current then flows from the center potential electrode 15 to the two end potential electrodes 11 and 12, in opposite directions.

- When the currents are made to flow through the two parallel circuits a and b in opposite directions, they will normally be of unequal strength and will therefore cause different potential drops across the identical resistances 17 and 18. This difierence will be indicated in the current respon- During the investigation of a section of a tract,

the apparatus above described may be operated as follows: Resistances 4 and 5 are set at arbitrary values. The potential drop between electrodes 11 and 15 causes a flow of current in the circuit a; and the potential drop between electrodes 15 and 12 causes the flow of current in the circuit b.

By properly adjusting the resistance 6, if necessary, and the rotor 16 of the variometer 9, the resistances of the circuits or and b may be so balanced that the same current flows in both of these circuits in opposite directions. The current responsive means 19 will of course indicate when the balanced condition has been obtained. When this balanced condition is obtained, the ratio between the values of the resistances 4 and 5 are dicative of the ratio between the ground potentials under investigation, as will be more fully explainedbelow.

The distance between the electrodes, including both the power electrodes and the potential .electrodes, or other equivalent pick-up means, may be chosen arbitrarily. In the present preferred practice of the invention, the center potential electrode is placed primarily within the influence of only one of the power electrodes, say electrode 2, and in a line between the two power electrodes, as indicated in the drawings. trodes 11 and 12 may be arbitrarily located away from. the center potential electrode 15. In the present preferred practice of the invention, the three potential electrodes are placed substantially in a straight line with respect to one another. Moreover, the end electrodes are preferably symmetrically spaced with respect to the center electrode.

In order to make a potential drop ratio determination for the potential drops between electrodes 15 and 11 and between electrodes 15 and 12, which may for convenience be designated as V1511, and Vl5-12, the potential electrodes 11, 15 and 12 are connected to the compensating network by means of the triple-pole double-throw switch 10, as shown. When the resistances 4 and 5 are set at arbitrary values, a noticeable deviation of the zero indicator 19 is generally produced. This deviation may be neutralized by appropriate manipulation of the sliding contact arm on the variable resistance 6, if necessary, and of the rotor 16 of the variometer 9. The resistance values are then noted.

The following equations may be considered as representing voltage conditions when no current flows through the indicator 19; that is, when a balanced condition has been obtained.

(1) V1s-11=i(R11+Z'1+Re+R4+R1'1-{- 2(Z1s+R,1s) +2a'21rfm) (2) V15-12=i(Riz+Zs+R5+R1a+ 2(Z1s-i-R15) 27'21rfm) Where Vi5-11=the potential drop between the points 15 and 11; V15-12=-the potential drop between the points 15 and 12; i=the net-work current 'flowing through electrode llor electrode 12; R==the index resistance of the apparatus elements indicated by the index numeral. Z=the index impedance of the apparatus elements indicated by the index numeral; I m=the mutual inductance between the ro-' tor and stators; I f=the frequency of the current source; and a and 1r=tW0 well known mathematical symbols.

Now, changing the setting of resistance 4 (or 5) and re-establishing neutralization by resetting resistance 5 (or 4), and rotor 16-until no current flows through the indicator 19, without touching any of the other adjustments, we have the following equation for voltage conditions Dividing Equation (1) by Equation (2) and Equation (3) by Equation (4), the following two equations may be obtained for the center branch resistance:

The two end elec- If Equations (5) and (6) are now combined, the following complex equation potential may be obtained for the potential drop ratio:

Excepting under unusual operating conditions, the factor containing the mutual inductance difference (m'-m) is very small compared with the resistance difierence (R4R4) and (R's-R5) and may-be neglected without causing an appreciable error. The final result is then given by the simplified equation:

This relation is extremely simple and contains only the resistance values essential for the computation of the potential drop ratio.

If for any reason it is desired to use the complete Equation (7) for the determination of the potential drop ratio, then this may be easily done by transforming-Equation (7) into an expression suitable for numerical computation, which can easily be done by anyone skilled in the art. However, as stated, Equation (8) is much more satisfactory for practical purposes and hardly less reliable. The above explained derivation of potential drop ratios may be carried out with a ground field of either direct or alternating current. When direct current is employed, it is not necessary to change the position of the variometer 9 to balance the constants of the circuits at and b, the balancing being eflected by the use of the resistances only. The type of indicating instrument 19 employed will depend upon the ground field current,

a galvanometer or ammeter being used with direct current ground fields.

The same procedure is used for the determination of electromagnetic field strength ratios, the compensating network then being connected to receiving coils 13 and 14 by suitably moving the switch 10. In this case, alternating current is employed, since direct current is not applicable with pick-up coils. If H13 and H14. are field strengths inducing the electromotive forces E15 and E14 in the coils for certain positions of the pick-up coils 13 and 14, then we have the following ratio:

H13 E13 7 (9) H14 E14 which may numerically be determined by Equation ('7) or (8).

The sensitivity of the apparatus may be considerably increased if deemed desirable by employing suitable-and well-known means to amplify the current imput to the indicator 19.

In order to carry out geophysical investigations, the electrode arrangement 2, 11, 15 and 12 may either be moved as a unit over the area to be investigated and readings taken at different points, or the potential electrodes 11, 15 and 12 may be moved independently in any desired direction; the same of course being true of the pick-up coil arrangement l3 and 14. In the present preferred practice of the invention, the conductive network including the pick-up means is moved from point to point within the influence of exciter elec- 4 trode-2. A series of readings are advantageously taken for each grounded position of power electrode 2 by moving thepotential electrodes 11, 15 and 12 from position to position, the center electrode 15 being preferably in line between efiect is obtained when using the contact arm 25 potential drop V25 as the reference potential, provided the sliding contact arm 25 is adjusted to a suitable potential value; and the single-pole double-throw switch 26 is moved to cut out center potential electrode 15. The compensating or balancing procedure with respect to the currents set up in the parallel circuits a and b of the conductive net-work and the computation of the resulting potential drop ratio is identical with the procedureand computation described for the potential drop ratio In a similar manner, either potential electrode 11 or potential electrode 12 may be replaced by, an artificial potential, obtained as shown in Fig. 2 by means of sliding contact arm 25 on the potentiometer 21. In that case single-pole doublethrow switch 23 or 24 is moved to cut out one of the electrodes and to make connection between its circuit in the compensating net-Work and the potentiometer 21. If the contact arm 22 is adjusted to a suitable potential drop Value, then the comparison and computation procedures are identical with those just outlined with respect to cutting out the center potential electrode 15.

Such an arrangement has the advantage that it reduces to a minimum the number of potential obtainable when moving a. fixed electrode arrangement over an area or tract to be investigated, the center potential electrode being symmetrically placed in a substantially straight line from the two end electrodes 11 and 12, and the center electrode 15 having a higher potential drop than either of the end potential electrodes. In this manner, each successive measurement will always be comparable because each succeeding determination is made from the same electrode distance arrangement. Using this electrode distance for successive tracts of substantial equal resistance, the ratio values are relatively small and uniform, and increase considerably above a better conductor.

Assuming that the ground contains a highly conductive ore body 2'7 and that the potential drop ratio profile. crosses the ore body at substantially ducting ore body will change in the manner diagrammatically illustrated by the curve 29. The electrical indications obtained by this arrangement are symmetrically located with reference to the actual location of the better conducting body, thereby permitting an accurate determination of its location.

Each point on the curve 29 represents a potential drop ratio determination with respect to the center potential electrode 15, as the exciter and pick-up devices are progressively moved over the tract. When the center potential electrode 15 is grounded in the immediate neighborhood of the ore body 27, the crest of the curve reaches an apex. As the center electrode is grounded over the top or tip of the ore body, the curve drops sharply as indicated. On the other hand, when the center electrode is grounded slightly beyond the ore body, the curve again reaches an apex. As the center electrode, together with the two end potential electrodes, always symmetrically spaced by the same distance, are progressively moved away from the ore body, the curve again flattens out.

I claim: j

1. In a method of electrical prospecting, the steps which comprise setting up and maintaining a given electric ground field in a tract to be investigated, producing an opposite current flow in two parallel circuits each of which has at least one variable resistance by the ground field intensities of different portions of said tract so that the current in either of said circuits flows clockwise when the flow in the other is counterclockwise, setting the variable resistance of one of said circuits at an arbitrary value without making any other adjustments, balancing the circuits by suitably adjusting the variable resistance in the other circuit until the same current flows through both circuits, noting the values of the variable resistances, altering the variable resistance of one of 'said circuits to another arbitrary value without making any other adjustment to produce a different current flow therethrough, again balancing said two circuits by suitably adjusting the variable resistance in the other circuit until the same current flows through both circuits, and noting the new values of the variable resistances.

2. In a method of electrical prospecting, the

- steps which comprise setting up and maintaining a given electric ground field in a tract to be investigated, producing an opposite current flow in two parallel circuits each of which has at least one variable resistance by the ground field intensities of different portions of said tract, so that the current'in either of said circuits flows clockwise when the flow in the other is counterclockwise, setting the variable resistance of one of said circuits at an arbitrary value without making any other adjustments, balancing the circuits by suitably adjusting the variable resistance in the other circuit until the same current flows through both circuits, noting the values of the resistances, altering the variable resistance of one of said circuits to another arbitrary value without me .ng any other adjustment to produce a different current flow therethrough, again balancing said two circuits by suitably adjusting the variable resistance in the other circuit until the same current flows through both circuits in the same direction,,and noting the new values or the resistances. I

3. In a method of electrical prospecting the steps which comprise setting upand maintaining a given electric ground field in a tract to be in-- vestigated, producing a current flow in a circuit having at least one variable resistance by the difference in potential between two spaced points in a tract, producing an opposite current flow in a second circuit having at least one variable resistance and a branch in common with said first circuit by the difference in potential between one of said two points and a third point in said tract, setting the variable resistance of one of said circuits at an arbitrary value without making any other adjustment, balancing the circuits by suitably adjusting the variable resistance in the other circuit until no current flows through the common branch, noting the values of the resistances, altering the variable resistance of one 0! said circuits to another arbitrary value without making any other adjustment to produce a. d11- ferent current flow therethrough, again balancing said two circuits by suitably adjusting the variable resistance in the other' circuit until no current flows through the common branch, and noting the new values of the variable resistances.

4. In a method of electrical prospecting, the steps which comprise setting up and maintaining a given electric ground field in a tract to be investigated, producinga current flow in a circuit having at least one variable resistance by the difference in potential between two spaced points in a tract, producing an opposite current flow in a second circuit having at least one variable, resistance and a branch in common with said first circuit by the difference in potential between one of said two points and a third point in said tract, setting the variable resistance of one of said circuits at an arbitrary value without making any other adjustment, balancing the circuits by suitably adjusting the variable resistance in the other circuit until the same current flows through both circuits, noting the values of the variable resistances, altering the variable resistance of one of said circuits to another arbitrary value without making any other adjustment to produce a difierent current flow therethrough, again 1091- ancing said two circuits by suitably adjusting the variable resistance in the other circuit until the same current flows through both circuits, and noting the new values of the variable resistances,

5. In a method oi. electrical prospecting the steps which comprise'setting up and maintaining a given alternating electric ground field in a tract to be investigated, producing an alternating current flow in two parallel circuits each of which has at least one variable resistance by the ground field intensities of difierent portions of said tract, the current flowing through the circuits in opposite directions so that the current in either of said circuits flows clockwise when the flow in the other of said circuits is counterclockwise, setting the variable resistance of one of said circuits at an arbitrary value without. making any other adjustment, balancing the circuits by suitably adjusting the variable resistance in the other circuit until the same current flows through both circuits, noting the values of the variable resistances, altering the variable resistance of one of said circuits without making any other adjustment to another arbitrary value to produce a different current flow therethrough, again balancing said two circuits by suitably 'adjusting the variable resistance in the other circuit until the same current flows through both circuits, and noting the new values of the variable resistances.

steps which-comprise setting up an alternatingthe other circuit until the same current flows electric ground field in a tract to be investigated, producing an alternating current fiow in a circuit having at least one variable resistance by the difference in potential between two spaced points in the tract, producing an alternating current fiow in a second circuit having at least one variable resistance and a branch in common with said first circuit by the difference in potential between one of said two points and a third point in said tract, the current flowing through the circuits in opposite directions, setting the variable resistance of one of said circuits at an arbitrary value without making any other adjustment, balancing the circuits by suitably adjusting the variable resistance in the other circuits until the the other circuit 'until the same current flows through both circuits, and noting the new values of the variable resistances.

7. In a method of electrical prospecting, the

. steps which comprise setting up an electric ground field with direct current in a tract to be investigated, producing a direct current flow in two parallel circuits each of which has at least one variable resistance by the ground field intensities of different portions of said tract, the current flowing through the circuits in opposite directions so that the current in either of said circuits flows clockwise when the flow in the other of said circuits is counter-clockwise, setting the variable resistance of one of said circuits at .an arbitrary value without making any other adjustment, balancing the circuits by suitably adjusting the variable resistance in the other circuit until the same current fiows through both circuits, noting the values of the variable resistances, altering the variable resistance of one of said circuits without making any other adjustment to another arbitrary value to produce a different current fiow therethrough, again balancing said two circuits by suitably adjusting the variable resistance in the. other circuit until the same current flows through both circuits, and noting the new values of the variable resistances. 8. In a method of electrical prospecting, the

. steps which comprise setting up an electric ground field with direct current in a tract to be investigated, producing a direct current flow in a circuit having at least one variable resistance by the difference in potential between two spaced points in the tract, producing a direct current fiow in a second circuit having at least one variable resistance and a branch in common with said first circuitby the difference in potential between of said two points and a third point in said tract, the current flowing through the circuits in opposite directions, setting the variable resistance of one of said circuits at an arbitrary value without making any other adjustment, balancing the circuits by suitably adjusting the variable resistance in the other circuit until the same current flows through both circuits, noting the values ofthe variable resistances, altering the variable resistance of one of said circuits without making any other adjustment to another arbitrary value to produce a diilerent current flow therethrough, again balancing said two circuits by suitably adjusting the variable re i tance in through both circuits, and noting the new values of the variable resistances. 1

9. In a method of electrical prospecting, the steps which comprise setting up an electric ground field in a tract to be investigated, producing a current flow in a circuit having at least one variable resistance by the difference in potential between-two spaced potential electrodes grounded in the tract, producing a current flow in a second circuit having at least one variable resistance and a branch in common with said first circuit by the difierence in potential between one of said two electrodes and a third potential electrode grounded in said tract, the current flowing through the circuits in opposite directions, setting the variable resistance of one of said circuits at an arbitrary value without changing the posi-' tion of the electrodes, balancing the circuits by suitably adjusting the variable resistance in the other circuit until the same current flows, through bothcircuits, noting the values of the variable resistances, altering the variable resistance or one of said circuitsto another arbitrary value withoutmaking any other adjustment to produce a different current flow therethrough, again balancing said two circuits by suitably adjusting the variable resistance in the other circuit until field in a tract to be investigated by a source of Y current connected to two spaced and grounded power electrodes, producing a. current flow in a circuit having at least one variable resistance by the difierence in potential between two grounded potential electrodes spaced from one of said power electrodes in the tract, producing a current flow'in a second circuit having at least one variable resistance and a branch in common with said first circuit by the difference in potential between one of said two potential electrodes and a third potential electrode in said tract, the current flowing through'the circuits in opposite directions, setting the variable resistance of one of said cir: cuits at an arbitrary value without making any other adjustment, balancing the circuits by suitably adjusting the variable resistance in the other circuit until the same current fiows through both circuits, noting the values of the variable resistances, altering the variable resistance of one of said circuits to another arbitrary value without making any other adjustment to'produce a different current fiow therethrough, again balancing said two circuits by suitably adjusting the variable resistance in the other circuit until the same current flows through both circuits, and noting,

having at least one variable resistance by an in duction or receiving coil, producing a current flow in a second circuit having at least one variable resistance and a branch in common with said first circuit by a second induction or receiving coil, the current flowing through the circuits in opposite directions so that the current in either of said circuits 'fiows clockwise when the flow in the other is counter-clockwise, setting the variable resistance of one of said circuits at an arbitrary value without making any other adjustment, balancingthe circuits by sui ably adjusting the variable resistance in the other circuit until the same current flows through both circuits, noting the values of the variable resistances, altering the variable resistance of one of said circuits to another arbitrary value to produce a different current flow therethrough, again balancing said two circuits by suitably adjusting the variable resistance in the other circuit until the same current flows through both circuits, and noting the new values of the variable resistances.

13. The method of electrical prospecting according to claimlO, in which the current produced in the circuits is effected by placing the center potential electrode closer to the power electrode than the other two potential electrodes so that the potential at the center electrode is greater than that at the other two potential electrodes. 7

14. The method of electrical prospecting according to claim 10, in which the potential electrodes are so placed with respect to the power electrodes that current flows from the center potential electrode to the other two potential elecno a 

